docs: def value(self, loss_type, time_event=None):
          """
          :returns: the total asset value for `loss_type`
          """
          if loss_type == 'occupants':
              return self.values['occupants_' + str(time_event)]
          try:  # extract from the cache
              val = self._cost[loss_type]
          except KeyError:  # compute
              val = self.calc(loss_type, self.values, self.area, self.number)
              self._cost[loss_type] = val
          return val